The present invention relates to processing data in a satellite positioning system, such as a Global Positioning System (GPS), and more particularly relates to methods and apparatuses which distribute the information processing and usage of GPS data.
GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on clock timing, so-called "ephemeris" data. The term "ephemeris" or "satellite ephemeris" is used to mean a representation, such as an equation, which specifies the positions of satellites (or a satellite) over a period of time or time of day. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, and results in significant power drain, especially for hand-held type units.
There are two principal functions of GPS receiving systems: (1) computation of the pseudoranges to the various GPS satellites, and (2) computation of the position of the receiving platform using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are simply the time delays measured between the received signal from each satellite and a local clock. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. As stated above, collecting this information normally takes a relatively long time (18 seconds to several minutes) and must be accomplished with a good received signal level in order to achieve low error rates.
Most GPS receivers utilize correlation methods to compute pseudoranges. These correlation methods are performed in real time, often with hardware correlators. GPS signals contain high rate repetitive signals called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A codes, and have a binary phase-reversal rate, or "chipping" rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 msec. The code sequences belong to a family known as Gold codes. Each GPS satellite broadcasts a signal with a unique Gold code. Alternative methods, as exemplified in U.S. Pat. No. 5,663,734, operate on snapshots of data and utilize fast convolution methods to compute the pseudoranges.
All of the above systems may benefit by communicating with the resources of a remote site, or "server" utilizing a wireless communications system, such as a cellular telephone system. Such a server may provide assistance data to the mobile GPS receivers to enhance their performance, it may receive data from the GPS receivers and perform further processing on such data to complete or refine a position calculation, or it may do both. In addition, the remote site may include various display and application resources, for example, dispatching means to send emergency or repair resources to the user of the GPS mobile, or to provide route guidance or other concierge services.
Thus, the above server provides two functions: (1) Location Server functions, which provide assistance to the mobile GPS receivers to enhance their performance, and (2) Application Server functions, which display the location of the mobile GPS receiver and provide auxiliary services, such as roadside assistance.
A paper was provided by Raab in 1977 on splitting the functionality of GPS processing between mobile GPS receivers and a remote basestation. See Raab, et al., "An Application of the Global Positioning System to Search and Rescue and Remote Tracking," Navigation, Vol. 24, No. 3, Fall 1977, pp. 216-227. In one method of Raab's paper the remote GPS receiver computes the times of arrival of the satellite signals at the remote GPS receiver (so-called "pseudoranges") and transmits these times-of-arrival to a central site via a data relay where the final position calculation of the mobile is computed. Raab also talks about providing assistance information including approximate time and position to the remote unit. Raab also discusses so-called "retransmission methods" in which the raw GPS signal is relayed directly to the remote basestation.
Other patents, such as U.S. Pat. Nos. 4,622,557, 5,119,102, 5,379,224, and 5,420,592 discuss variations of the retransmission method. U.S. Pat. No. 4,622,557 utilizes an analog retransmission method whereas U.S. Pat. Nos. 5,119,102, 5,379,224, and 5,420,592 utilize digital means to store and then forward a digitized record of the sampled GPS signal. These patents describe communications between one or more mobile units and a single basestation which may incorporate functions of GPS calculation as well as ancillary functions described above.
The U.S. Pat. No. 4,445,118 by Taylor discusses transmission of aiding data, such as GPS satellites in view from a basestation to remote units via a communication link. In addition, in one variation, a tracking application for trucks, Taylor describes a system in which pseudorange data is sent from the trucks to the remote basestation which computes the final position. Variations on this pseudorange transfer method include U.S. Pat. Nos. 5,202,829 and 5,225,842. Again, this prior art envisioned a single basestation containing GPS aiding functions as well as display and other ancillary functions.
FIG. 1 shows a block diagram of the prior art which utilizes a basestation to supplement GPS signal processing. Mobile units 12a, 12b, 12c, and 12d in this example contain a combination of a GPS receiver and a wireless modem. Attached to the GPS unit are GPS antennas 10a, 10b, 10c, and 10d for receiver GPS signals from GPS satellites (not shown for simplicity) and antennas 11a, 11b, 11c, and 11d for communication to and from a basestation 20 which includes a basestation antenna 17. In some implementations, this communication may be in one direction only.
Basestation 9 contains a signal processing unit 15 which may provide aid to the mobile GPS units to help them obtain positioning information and/or it may complete or refine the position calculations of these units based upon data transmitted to it from these units, together with auxiliary data which it may gather with its own GPS antenna 18. The signal processing unit 15 may contain its own GPS receiver and GPS antenna in order to determine its own position and provide differential corrections to the data transmitted to it from the mobile GPS units. Basestation 9 also includes a display 14 and computer equipment which is coupled to the signal processing unit 15 by a connection 16 and which allows an operator to visually track the position of the mobiles and provide manual and semiautomatic commands to these units via the aforementioned communications links. In some cases, unit 14 together with signal processing unit 15 is termed a "workstation."
Although FIG. 1 shows a wireless link from each mobile GPS unit to the basestation, this link may actually be a wireless link to a modem, such as one at a cell site followed by a wired or other link to the basestation as shown in FIG. 1. The important thing to note is that the configuration of FIG. 1 consists of a single basestation site and this basestation includes both the functions of GPS assistance and applications support (e.g. position monitoring, dispatching, etc.).
There are many limitations in the implementation of FIG. 1. For example, utilizing a single workstation in this "star" configuration means that there may be long path delays from each of the GPS mobile units to and from the basestation.